1. Field of the Invention
The invention relates to a rotating electrical machine cooling system, and more particularly, to a rotating electrical machine cooling system capable of cooling a rotating electrical machine with the aid of a mechanical coolant pump and an electric coolant pump.
2. Description of Related Art
In a vehicle that is mounted with an engine and a rotating electrical machine, a so-called electric type oil pump (an electric oil pump) as well as a so-called mechanical type oil pump (a mechanical oil pump) is employed in order to cool the rotating electrical machine, an automatic transmission, and the like. The mechanical oil pump is driven by the engine. The electric oil pump is driven by a battery or the like even when the engine is stopped.
For example, in Japanese Patent Application Publication No. 2006-161851 (JP-2006-161851 A), there is described a configuration of a vehicular oil pressure supply device that is equipped with a mechanical oil pump and an electric oil pump. In this configuration, a check valve is provided between a discharge side of the mechanical oil pump and a discharge side of the electric oil pump in order to prevent hydraulic oil from flowing backward from the mechanical oil pump to the electric oil pump. Furthermore, an orifice and a relief valve are provided between a discharge-side oil channel and a suction-side oil channel, sequentially in the direction from the discharge-side oil channel to the suction-side oil channel. The discharge-side oil channel connects the discharge side of the mechanical oil pump and the discharge side of the electric oil pump with each other. The suction-side oil channel connects a suction side of the mechanical oil pump and a suction side of the electric oil pump with each other.
In Japanese Patent Application Publication No. 2011-978 (JP-2011-978 A), there is disclosed an auxiliary pump drive control device capable of driving an electric oil pump at low load even if the viscosity of oil becomes high when the temperature of outside air is low, in a vehicle that is equipped with a mechanical oil pump that is driven together with an engine, and the electric oil pump, which is an auxiliary pump that is driven by a pump drive source that is different from the engine. In this case, the mechanical oil pump and the electric oil pump are provided in parallel with each other between an oil pan as an oil store portion and a main discharge oil channel. A main check valve is provided between a discharge side of the mechanical oil pump and the main discharge oil channel. An auxiliary-side check valve is provided between a discharge side of the electric oil pump and the main discharge oil channel. In addition, apart from an auxiliary suction oil channel between a suction side of the electric oil pump and the oil pan, a subsidiary suction oil channel is provided between the discharge side of the electric oil pump and the oil pan via a check valve.
In addition, when the temperature of outside air is low, the electric oil pump is first reversely driven. Thus, high-viscosity oil that has remained in the auxiliary suction oil channel to become low in temperature is returned to the oil pan. Because the oil stored in the oil pan has been relatively warmed, the oil temperature rises in this oil pan. Besides, oil is sucked from the oil pan via the subsidiary suction oil channel by the electric oil pump, and is returned to the oil pan via the electric oil pump. This oil is sucked from the warmed oil pan, so that load on the electric oil pump can be kept low. By continuing this process, the oil temperature rises. After that, the electric oil pump is caused again to rotate positively. It is described in the aforementioned publication that the load of the electric oil pump can thus be lightened even when the temperature of outside air is low.
In the case where the mechanical oil pump and the electric oil pump are connected in parallel to each other to cool the rotating electrical machine, even when a check valve is provided between both the oil pumps, the coolant may flow backward if there is a certain relationship in magnitude between the discharge pressure of the mechanical oil pump and the discharge pressure of the electric oil pump. For example, when the coolant flows backward from the mechanical oil pump to the electric oil pump, the electric oil pump rotates reversely. Besides, the coolant that has flowed backward may leak out. When the electric oil pump rotates reversely, an electric motor that drives the electric oil pump rotates reversely to assume a regenerative state, so that a drive circuit of the rotating electrical machine may be damaged. Incidentally, when the coolant flows backward from the electric oil pump to the mechanical oil pump, the mechanical oil pump does not rotate reversely because it is connected to the engine. Besides, since the coolant that has flowed backward is returned from a suction port to a coolant reservoir, the coolant does not leak out either. As described hitherto, there is a problem in a structure in which only the check valve is employed in the case where the mechanical oil pump and the electric oil pump are connected in parallel to each other to cool the rotating electrical machine.
In order to avoid this problem, it is conceivable to perform control of driving only one of the mechanical oil pump and the electric oil pump such that the driving of the mechanical oil pump and the driving of the electric oil pump do not compete against each other. In this case, if only one of the oil pumps is driven, it may be impossible to supply a sufficient amount of the coolant for cooling the rotating electrical machine. If an attempt is made to sufficiently cool the rotating electrical machine by driving only one of the oil pumps, that one of the oil pumps is enlarged in size.
Besides, it is assumed that the temperature of the coolant is lowered by a heat exchanger such as an oil cooler or the like in the case where the mechanical oil pump and the electric oil pump are connected in parallel to each other to cool the rotating electrical machine. Then, the coolant flows via the oil cooler at the time of extremely low temperature as well, so that it is difficult to raise the temperature of the coolant. In general, at the time of low temperature, the temperature of the coolant is raised by operating the electric oil pump, which can be driven by a battery. In this situation, while the engine is stopped and the backflow of the coolant is prevented by a check valve, the electric oil pump can be operated. In this case as well, however, the coolant from the electric oil pump flows via the oil cooler. Therefore, the process of raising the temperature of the coolant is inefficient.